One example of a rotary machine for an aircraft is the turbofan, gas turbine engine. The engine includes a compression section, a combustion section, and a turbine section. A primary flowpath for working medium gases extends axially through these sections of the engine. The flowpath is annular in shape. A stator assembly extends circumferentially about the annular flowpath to bound the flowpath. A rotor assembly extends axially through the engine inwardly of the casing between the compression section and the turbine section.
Under operative conditions, working medium gases are drawn into the compression section. The gases are passed through several stages of compression, causing the temperature and pressure of the gases to rise. The pressurized gases are mixed with fuel in the combustion section and burned to form even hotter, pressurized gases. These gases are a source of energy to the engine and are expanded in the turbine section to produce work.
A secondary flowpath for working medium gases is disposed outwardly of the primary flowpath. The secondary flowpath is also annular in shape. The secondary flowpath is bounded by an outer duct wall and an inner duct wall. The inner duct wall of the secondary flowpath may provide an outer casing which extends circumferentially about the sections of the engine. One example of such a turbofan engine is the JT8D model turbofan engine manufactured by Applicant's assignee. In the JT8D engine, a second or inner case is spaced radially inwardly of the outer casing (inner duct wall) and provides the pressure vessel for containing the working medium gases on the interior of the engine. As a result, the inner casing has a circumferential hoop-strength characteristic which is greater than the circumferential hoop-strength characteristic of at least a portion of the outer casing. The hoop-strength characteristic is a measure of the ability of the casing to resist circumferential stresses. In this construction, a circumferential bracket extends between the inner case and the outer case and is integrally attached to both to positively support the outer case from the inner case.
The rotor assembly is used to transfer energy from the hot working medium gases of the primary flowpath in the turbine section to the compression section. In the compression section, the energy is transferred to working medium gases drawn into the compression section to compress the incoming gases. The rotor assembly includes a rotor disk in the turbine section and arrays of rotor blades which extend outwardly across the working medium flowpath in the turbine section to receive energy from the gases. The expanding gases in the turbine section are flowed over the radially extending rotor blades and drive the rotor assembly about the axis of rotation. Arrays of rotor blades in the compression section extend outwardly from the rotor assembly and are driven by the rotor assembly about the axis of rotation to compress the working medium gases drawn into the compression section.
Heat is transferred from the high-temperature gases in the turbine section to components in the turbine section, such as the inner case and, to a lesser extent, to the outer case. The temperature in the outer case, which is remote from the hot working medium gases and to some extent cooled by the passage of gases in the secondary flowpath over the outer case, is much less than the temperature of the inner case.
The inner case expands outwardly toward the outer case because of the difference in temperature between the inner case and the outer case, and the higher coefficient of thermal expansion of the inner case in comparison with the outer case. The difference in thermal growth has caused cracking of the support bracket which extended between the inner case and the outer case and was rigidly attached to both in early constructions of the JT8D engine.
One solution to the cracking problem while providing positive support to the outer casing (or inner duct) from the inner casing was to provide a spline-type connection between the inner case and the outer case. This permitted the inner case to grow radially with respect to the outer case while positively supporting the outer case with the inner case through the spline-type connection.
Positively supporting the outer case from the inner case increases the capability of the gas turbine engine to contain blade failures. In axial-flow rotary machines, the rotor assemblies are driven at high rotational speeds about the axis of rotation. Occasionally a blade may fail, for example, when a foreign object is ingested into the engine or the fatigue life of the blade is exceeded. During such a failure, pieces of the rotor assembly may be hurled outwardly from the rotor assembly with velocities of several hundred feet per second. The inner case and the outer case in such circumstances must stop the particles to avoid having the particles from the rotor assembly cause damage to other portions of the engine.
The above art notwithstanding, scientists and engineers working under the direction of Applicant's assignee have sought to improve the durability and containment capability of these casing structures.